Electroacoustic transducer

ABSTRACT

The plate piston, vibratile diaphragm of an underwater transducer is divided by grooves to form a mosaic of piston element islands interconnected by a thin web. Each piston element is separately driven by an individually associated transducer element. The transducer elements may be coupled electrically to enable associated equipment to make a phase comparison which gives directional information. The invention has particular utility when the diameter of the total plate piston is greater than the wave length of the sound radiated into the water.

United States Patent [72] In en Fl'lnk M158 1'" 2,748,369 5/1956 Smyth 340/ Cohasset. Mass. 2,912,856 11/1959 Kirtz 340/10 X [21] Appl. No. 737,198 2,921,288 1/1960 O'Neill et a1 340/9 X 122] Filed June 14, 1968 2,979,690 4/1961 Hackley t r r 340/8 [45] Patented July 13,1971 3,284,761 11/1966 Douglas 340/10 [731 Assignee Massa Division.1)ynamics Corporation 3,328,751 6/1967 Massa r. 340/10 of America Mass Primary Examiner- Rodney D. Bennett, Jr.

Assistant Examiner-Biran L. Ribando [S4] ELECTROACOUSTIC TRANSDUCER A o neys ernat Claims, 6 Drawing Figs.

[52] 11.5. CI. 340/9, ABSTRACT; The plate piston. vibragile diaphragm f an 340/101 340/14 derwater transducer is divided by grooves to form a mosaic of III. H041 i ton elgmcnl islands interconnected by a thin web, Each 1 new olsflllch 340/33 piston element is separately driven by an individually as- 14 sociated transducer element. The transducer elements may be coupled electrically to enable associated equipment to make a [56] utterances Cited phase comparison which gives directional information. The in- UNITED STATES PATENTS vention has particular utility when the diameter of the total 2,063,951 12/1936 Steinberger .4 340/9 plate piston is greater than the wave length of the sound 2,181,132 11/1939 Kallmeyer 340/10 radiated into the water.

I 46 A 2 l I A d l 46 T PATENIEI] Jun 3 ISYi SHEH 1 OF 2 INVENTOR FRANK M4554 Ii! ELECTROACOUSTIC TRANSDUCER This disclosure is similar to the disclosure in a copending U.S. application Ser. No. 720,l47, filed Apr. l0, l968.

This invention relates to acoustical transducers and more particularly to underwater transducers having diameters which are larger than the inwater wavelength of the sound which they radiate.

For present purposes, transducers of the described type may be thought of as parts of sonar homing equipment; although the invention is not limited to use with such equipment. The transducers must radiate sonic energy in a very sharp directional beam or pattern free of excessive side lobes. The transducer should have characteristics that enable the associated equipment to steer an underwater sound beam or to sense the direction of reflected sonic energy. This way, a robot craft-such as a torpedo-may carry a scanning sonar beam for detecting a target, and then the craft may turn toward and home upon the target.

Sometimes these homing systems require transducers having a total radiating surface with a diameter which is larger than the wave length of the radiated sound so that the beam will be sharp and directional. If the diameter of this surface were comparable to or smaller than the wave length of the radiated sound, the beam pattern would be very broad or omnidirectional. However, transducers of large dimensions using simple, unshaded pistons tend to have side lobes which are objectionably large.

it becomes diflicult to make a transducer of the described type when the radiating element is large. When. the radiating surface is small enough to operate in the broad beam or omnidirectional range, a single electroacoustical vibratory element is satisfactory. However, if attempts are made to use the same general type of structure in the larger transducers, transverse flexural modes of vibration occur in the plate piston to disturb the radiation pattern.

To overcome the problems of unduly large transducers, it is possible to provide a mosaic of small, separate elements joined into a single large plate insofar as radiated sonic energy is concerned. For example, U.S. Pat. No. 2,427,062 shows such a mosaic wherein the total surface is large, but the individual mosaic elements are small enough to operate efficiently. Nevertheless, the large plate continues to have the undesirable flexural or transverse modes of vibrations.

Accordingly, an object of the invention is to provide new and improved narrow beam electroacoustic transducers. in this connection, an object is to provide transducers for producing directional beam patterns. Further, an object is to provide transducers for generating separate directional beams within the same unitary structure.

Another object of the invention is to provide low cost transducers which are more efficient than comparable transducers heretofore available.

Yet another object of the invention is to provide symmetrical multiple beam patterns. Here an object is to provide beam patterns which are free of objectionable secondary lobes. Further, an object is to provide easily steerable acoustic beams. in particular, an object is to provide sonar transducers especially well suited to control robot crafts, such as homing torpedoes, for example.

In keeping with an aspect of the invention, an underwater transducer uses a vibratile plate piston subdivided into a number of piston element islands interconnected by a thin web. For example, the plate may be undercut by deep grooves to form a wafflelilte structure. A separate transducer element drives each piston element or island. Associated equipment may interpret the signals to individual transducer elements in order to derive directional information.

An exemplary embodiment of the invention may be understood best from a study of the following description and specification when read in connection with the accompanying drawings in which:

FIG. I is a schematic plan view looking down into the rear of an inventive transducer;

FIG. 2 is a cross-sectional view (including the rear lid which is not shown in FIG. 1) of the transducer taken along the line 2-2 of FIG. l;

FIG. 3 is a cross-sectional view of an alternative embodiment for use in deep water in which the transducer assembly is attached to the rear cover plate and acoustic energy is transmitted through a sound transmitting fluid in the housing structure;

FIG. 4 is a cross-sectional view of yet another deep water embodiment of the invention which avoids the necessity of filling the transducer with a coupling fluid;

FIG. 5 is a cross-sectional view of a detail showing an assembly of the electroacoustic vibrator mounted on one of the sectionalized portions of the vibratile piston surface; and

FIG. 6 graphically illustrates the reduction of the secondary lobes in the sonic beam pattern which is achieved by the invention.

A first embodiment of the invention (FIGS. 1 and 2) includes as its principal parts a housing structure 26, a flat vibratile sonic energy radiating surface 21, and a number of transducer elements 22. The housing 20 is a cup-shaped structure terminating at the bottom in the sonic radiating surface 21 and at the top in a recessed step 23. The diameter of surface 21 is greater than the wavelength of sound radiated from the surface. A suitable lid 24 rests upon the step 23 and gives a waterproof seal to the structure. The assembly is completed by bolts such as 25 which secure the lid to the housing 20.

The bottom 21 is undercut by several very deep grooves 26-28 which form the plate into a number of elementary piston islands interconnected by a thin web. While the depth of the grooves is not critical, they should preferably extend through approximately 60 percent of the thickness of the plate. These grooves are here shown as having created 12 separate islands which are held together by the web of material in the approximately 40 percent of the plate remaining at the bottom of the grooves. For easy identification, three of the elementary piston islands (30, 31, 32) are cross-hatched in FIG. 1.

The positions of the grooves are selected to provide approximately equal areas for each of the islands. in greater detail, the grooves include two concentric circles 26, 27 and four radial spoke grooves 28, 35, 36, 37. In addition, there are four partial spoke radial grooves 38, 39, 40, 41. The diameter of the smaller circle 27 is approximately 50 percent to 60 percent of the diameter of the outer circle 26. With this arrangement, the areas of each of the three islands, 30, 31, 32 are approximately the same.

By inspection, it should be apparent that the radiating surface is divided into four equal quadrant areas by the radial spokes 28, 35, 36, 37. One of the quadrants includes the crosshatched islands 30-32. It and the other three quadrants also have equal areas.

The web matrix joining the islands forms the entire unit into a single composite piston. At the operating frequency the compliance of the web is sufficient to acoustically disconnect each island from its neighbors. But, the strength of the web is also sufficient to maintain an integrated mechanical structure with no significant chances of failures because of metal fatigue occurring during the normal life time of the transducer when it is operated above a certain depth. Additional information on the subject of web compliance is found in my U.S. Pat. No. 3,319,2l9.

The transducer elements 22 are arranged so that a separate transducer element is individually associated with each elementary piston island. Each transducer includes a polarized ceramic ring, such as 43, which may be made from any suitable piezoelectric material, such as lead zirconate titanite. This material is polarized for operating in the thickness mode; however, other known types of polarization may also be used. Or, crystal plates may be used, as taught in U.S. Pat. No. 3,328,75 l.

The individual transducer element includes a laminated structure comprising an inertial mass 45, an insulating washer 46, a piezoelectric ceramic ring 43, and an insulating washer 47 attached to the vibratile piston element 31 by means of a bolt 48. To further secure the assembly, epoxy cement may be applied between inertial mass 45 and the washer 46, and between the washer 47 and the piston element 3|.

A pair of thin metal electrodes terminating in tabs 49, 50 are interposed between the insulating washers 46, 47 and the ceramic transducer element 43. The tabs 49, 50 provide connection points for the electrical wires used to energize the ceramic transducer elements. Again, epoxy cement may be used between the insulating washers 46, 47 and the electrodes including the tabs 49, 50. It should be understood that each of the 12 piston elements formed by the l2 islands is individually driven by a similar transducer.

The resonant frequency of each transducer element depends upon the stiffness of the ceramic ring 43 and upon the masses of the inertial element 45 and the island section 3!. Since the transducers are the same and since the areas and masses of each of the 12 islands are equal, the resonant frequencies of all driving elements are also equal. Any unwanted inequalities may be eliminated by changing the inertial mass associated with the nonstandard transducer. The change may be made by varying either the length or the diameter of the mass; however, the tops of the transducers will lie in a plane if only the diameter is changed.

To provide a means for completing the electrical circuits, a plurality of insulated terminals 53 are extended through the cover 24. This way, an electrical conductor (such as 52) may be extended from the terminal 53 to any suitable control equipment. Likewise, a conductor (such as 54) may complete the connection from the insulated terminal 53 to the transducer electrodes, such as tab 49, for example.

The exact network and manner of making electrical connections are not shown since they may take any of many different forms. For example, if the transducer is to operate as a narrow beam sound generator, all of the transducers may be connected in parallel. The directional beam pattern has a main beam angle, which is determined by the overall diameter of the sectionalized piston.

When the total diameter of the vibrating surface is greater than the wavelength of the sound radiated into the water, the beam pattern develops relatively large offensive secondary lobes, as shown at 56, 57in FIG. 6 by solid lines. [fall 12 of the transducers are connected in parallel to drive 12 equal area piston elements, the sonic intensity of the secondary lobes 56, 57 are about 17 db. lower than the maximum intensity of the main beam.

To reduce the relative strength of the side lobes, the amplitude of some piston sections are shaded. In greater detail, the transducers elements driving the two outer islands (e.g. 30,31) in each quadrant are connected in series. That series circuit involving the islands 30, 3! is then connected in parallel with the transducer element driving the inner island 32 in the same quadrant. Thus for example, the transducer elements driving islands 30, 3] are connected in series with each other and in parallel with the transducer element driving the island 32. Further, the radius of the inner circular groove 27 is about 50 percent to 60 percent of the radius of the outer circular groove 26. Therefore, the vibrating amplitude of the inner island 32 is about twice as great as the vibrating amplitude of the outer islands 30, 3!. The amplitude shading reduces the magnitude of the side lobes 58, 59 by db. or more. The total beam shape is changed somewhat as shown by dashed lines in FIG. 6. Therefore a preferable beam pattern may be achieved by making a simple electrical connection-which involves almost no cost.

To provide a directional sensitivity, the total piston plate 2! is divided into four quadrants. The transducer elements driving the islands forming elementary pistons of each quadrant are coupled as a unit. This way, suitable control equipment, of known design, may compare the phase of signals received at each of the quadrants with the phase of signals received at the other quadrants. The comparison device indicates whether the detected signal is coming in from the right, left, above or below an axis line normal to the radiating surface. If the detected signal is a sonar reflection, the indicated direction of the target may be used to drive a robot craft-such as a torpedotoward the target.

The principal advantage of the invention described thus far is that it is a very low cost structure. It is easy to construct, and it operates simply. However, it is limited in that it can not be operated at or below a depth which might rupture the web or permanently distort the radiating surface.

When it is desirable to operate the transducer in deeper water, it may be constructed as shown in FIGS. 3 and 4. FIG. 3 shows a transducer filled with a coupling liquid; FIG. 4 does not require such a liquid.

FIG. 3 shows a transducer comprising a cup-shaped housing 60, tenninated at one side by a resilient window 61 bonded to cup 60 and at the other side by a sturdy lid 62 held in place by a number of bolts 63. The resilient window could be rubber or any similar material. An "0" ring 64 is positioned between the lid 62 and the cup 60 to complete a waterproof seal of the transducer. A tapered, threaded plug 65 seals a port through which a coupling fluid 66 may be poured in order to fill the transducer. Any suitable fluid may be used, such as castor oil or silicone, for example.

The lid 62 and bolts 63 are strong enough to withstand deep water hydrostatic pressures. The resilient window 61 flexes to equalize the pressure inside and outside the transducer housing 60.

Inside the housing 60 is a sound radiating piston 68, divided into a number of elementary piston islands by a number of grooves. Again, as with the embodiment of FIG. 2, the islands are held together by a web of thin metal at the bottom of the grooves. Each of the piston elements is individually driven by an associated transducer element. A low acoustic impedance material 69 is interposed between the inertial masses and the lid 62. While different means may be used to make this attachment, I prefer to cement the assembly in place by an epox- A suitable waterproof connector 70 may be attached to the lid 62 in any suitable and known manner. The electrical connections are made as described above.

When the transducer is to operate in deep water, but a coupling liquid is not desired, the housing may be constructed as shown in FIG. 4. Here the structure is essentially the same as the structure of FIG. 3. However, instead of a plate piston suspended in a coupling fluid, this embodiment shows the piston 71 embraced by or floating in the resilient material 72 forming a sonic window bonded into the housing 73.

All three embodiments of the invention (FIGS. 2, 3, 4) operate in substantially the same manner, as described above.

The embodiments of FIGS. 3, 4 subject the ceramic transducer material to the hydrostatic pressure prevailing at the operating depth. This presents two problems. First, there is the mechanical problem that the transducer might be damaged, or even crushed, by the hydrostatic pressure. Second, the transducer would tend to display different operating characteristics at different depths.

To overcome these and other problems, the transducer may be constructed as shown in FIG. 5. Here, the same reference characters identify the parts which are the same as comparable parts shown in the other figures. The difference lies in the construction of the inertial mass 45.

In greater detail, the inertia] mass includes upper and lower parts 75, 76 which are of similar geometrical size and shape. The under side of part 75 and the upper side of the part 76 are recessed to provide an opening for receiving a pair of springs 77, 78. Each of these springs is a convex or cup-shaped spring washer positioned to push against each other and bias the inertial masses 75, 76 into a separated position. Here, the apex of each convex spring washer is placed to rest against the apex of the other convex spring washer. The bolt 48 helps secure the spring washers 77, 78 and inertial masses 75, 76 in place.

According to the invention, the static stresses acting upon the ceramic transducer are uniform regardless of the depth at which the transducer is operated. In greater detail, the bolt 48 is tightened until these stresses reach a predetermined value responsive to a compression of the spring washers 77, 78. As the hydrostatic pressure increases with depth, such increase is absorbed by the compliance of the springs. However, the net stress in the springs 77, 78 results in a reduced stress upon the bolt 48.

A moment's reflection should make it apparent that the several embodiments disclosed herein teach how the inventive transducer may be made at low cost and how that low cost structure may be modified to enable an operation at any of many different depths. Still other modifications will readily occur to those who are skilled in the art. Therefore, it should be understood that the appended claims should be construed to cover all equivalents reasonably falling within the true scope and spirit of the claims.

lclaim:

1. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, said islands having an average transverse dimension and covering substantially the entire plate, each of said islands forming a vibratile piston element having a thickness which is less than the average transverse dimension thereof, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.

2. The transducer of claim 1 wherein each of said grooves has a depth exceeding 50 percent of the depth of said plate.

3. The invention in claim 1 wherein said plurality of piston elements are arranged symmetrically about a center line normal to said vibratile plate.

4. The invention in claim 1 wherein said vibratile piston elements are arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate.

5. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said vibratile piston elements being arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, each of said symmetrical groups including a pair of said islands and their associated transducer elements spaced apart near the periphery of said plate and at a distance removed from the normal axis of said plate, a third of said islands and its associated transducer element located near the center of said plate and adjacent said normal axis, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.

6. The invention in claim 5 and means for energizing said transducer elements so that the electrical input power applied to each of a pair of transducer elements driving said peripheral plates is less than the power applied to a third transducer ele ment driving said center plate.

7. The invention in claim 5 wherein said transducer elements are piezoelectric material, said pair of transducer elements being electrically connected in series, said third transducer element being electrically connected in parallel with said series connected pair.

8. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands.

each of said islands forming a vibratile piston element, said vibratile piston elements being arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate, said symmetrical groupings comprising four identical quadrants, each of said quadrants containing a first piston element adjacent to the said normal axis and two other piston elements removed from said normal axis and symmetrically located about the outer periphery of said first piston element, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.

9. The invention in claim 8 wherein the sonic radiating areas of each of said piston elements has approximately the same surface area.

10. The invention in claim 8 wherein the transducers on the two other elements are electrically connected in series.

ll. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said vibratile plate being substantially circular, said grooves including two concentric circular grooves, the diameter of the smaller circular groove being approximately 50 percent to 60 percent of the diameter of the larger circular groove, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.

12. The invention of claim 11 and means for driving the piston elements lying within the area bounded by the smaller circular groove at an amplitude which is greater than the amplitude of elements lying in the region between said concentric grooves.

13. An underwater electroacoustic transducer comprising a housing structure including a peripheral wall portion terminated at one side by a resilient sound transparent window exposed to surrounding water, a vibratile plate portion mounted to drive sound waves through said window and into said water, said vibratile plate having a linear transverse dimension which is greater than the wavelength of sound radiated into the water at the operating frequency of said transducer, grooves in one surface of said plate dividing said plate into a plurality of acoustically separated islands, each of said islands having individual transverse dimensions which are greater than the thickness of said islands, each of said islands forming a vibratile piston element movable as an integral unit, said islands being held together mechanically by the thin weblilte structure remaining beneath the bottoms of said grooves, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, means for attaching said transducer elements in individually associated relationship to drive said separated vibratile piston elements, and means for electrically connecting said transducer elements to operate at a fixed relationship with respect to each other.

14. The invention in claim 13 wherein said vibratile plate portion is sealed to said peripheral wall portion by said resilient material.

15. The invention in claim 14 wherein said resilient material embraces said plate and completely covers the radiating surface of said vibratile plate.

16. An underwater electroacoustic transducer comprising a housing structure including a peripheral wall portion terminated at one side by a resilient sound transparent window exposed to surrounding water, a vibratile plate portion mounted to drive soundwaves through said window and into said water, said wall portion and said resilient window being a single unitary housing structure, said plate being suspended adjacent said resilient window, a coupling liquid completely filling said housing structure, said vibratile plate having a linear transverse dimension which is greater than the wavelength of sound radiated into the water at the operating frequency of said transducer, grooves in one surface of said plate dividing said plate into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said islands being held together mechanically by the thin weblilte structure remaining beneath the bottoms of said grooves, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, means for attaching said transducer elements in individually associated relationship to drive said separated vibratile piston elements, and means for electrically connect ing said transducer elements to operate at a fixed relationship with respect to each other.

17. An electroacoustic transducer comprising a housing having a sound transparent resilient wall portion with at least one linear dimension which is larger than the wavelength of sound radiated by said transducer when being driven at its frequency of operation, a plurality of grooves recessed into said vibratile wall portion for dividing said vibratile wall portion into a plurality of pistonlike vibratile surfaces, a plurality of electroacoustic transducer element means, each of said transducer elements having a mass element held against and separated from said housing by a low acoustic impedance material, means for attaching said electroacoustic transducer element means in operable relationship to individually associated ones of said pistonlilte vibratile surfaces, and electrical conductor means for interconnecting said electroacoustic transducer element means.

18. In combination in an electroacoustic transducer, a housing structure having a peripheral portion and a resilient end portion sealed to said peripheral portion to form a generally cup-shaped unit, the external area of said end portion being characterized in this that it presents a smooth and uninterrupted surface to the medium into which the transducer is operating, a vibratile plate, the inner surface of said end portion being adjacent one side of said vibratile plate, the other side of said plate being provided with a matrix of deep grooves which divide said vibratile plate into a plurality of acoustically separated vibratile piston elements held together by the thin weblike grid structure remaining at the bottoms of said deep grooves, each of said piston elements being thinner than its narrowest transverse dimension, a plurality of transducer elements for convening electrical signals into corresponding mechanical vibrations attached in operable relationship to drive individual ones of said separated vibratile piston elements, and means for interconnecting said plurality of transducer elements to form electrical operating units.

19. In combination in an electroacoustic transducer, is complex vibratile piston comprised of piston islands elements joined by an undercut web, each of said islands having a thickness which is less than the average transverse dimension of said islands, a plurality of electroacoustic transducer elements for converting electrical energy into mechanical energy, said elements being exposed to hydrostatic pressures surrounding said transducer, a plurality of inertial mass elements, means for attaching said mass and transducer elements in individually associated operable relationship, each of said transducer elements being mechanically bonded between individually associated piston islands elements and individually associated ones of said inertial mass elements so that axial vibrations of said elements may take place during vibration of said transducer elements, a plurality of stress means free of rigid attachments to a supporting structure for applying a predetermined compressive mechanical stress along a vibrational axis of individually associated ones of said transducer elements, each of said stress means including a rigid washerlike plate a pair of convex spring washers for neutralizing said h drostatic pressures, and mechanical fastening means for app ying a compressive force from said rtgld washer through said spring washers, to the inertial mass element and along the vibrating axis of said transducer element.

20. The invention in claim [9 characterized in that the transducer element is in the form of a hollow cylinder and further characterized in that said mechanical fastening means is a bolt which passes through the hollow cylinder. 

1. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, said islands having an average transverse dimension and covering substantially the entire plate, each of said islands forming a vibratile piston element having a thickness which is less than the average transverse dimension thereof, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.
 2. The transducer of claim 1 wherein each of said grooves has a depth exceeding 50 percent of the depth of said plate.
 3. The invention in claim 1 wherein said plurality of piston elements are arranged symmetrically about a center line normal to said vibratile plate.
 4. The invention in claim 1 wherein said vibratile piston elements are arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate.
 5. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said vibratile piston elements being arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, each of said symmetrical groups including a pair of said islands and their associated transducer elements spaced apart near the periphery of said plate and at a distance removed from the normal axis of said plate, a third of said islands and its associated transducer element located near the center of said plate and adjacent said normal axis, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.
 6. The invention in claim 5 and means for energizing said transducer elements so that the electrical input power applied to each of a pair of transducer elements driving said peripheral plates is less than the power applied to a third transducer element driving said center plate.
 7. The invention in claim 5 wherein said transducer elements are piezoelectric material, said pair of transducer elements being electrically connected in series, said third transducer element being electrically connected in parallel with said series connected pair.
 8. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said vibratile piston elements being arranged in symmetrical groupings about an axis normal to the plane of said vibratile plate, said symmetrical groupings comprising four identical quadrants, each of said quadrants containing a first piston element adjacent to the said normal axis and two other piston elements removed from said normal axis and symmetrically located about the outer periphery of said first piston element, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.
 9. The invention in claim 8 wherein the sonic radiating areas of each of said piston elements has approximately the same surface area.
 10. The invention in claim 8 wherein the transducers on the two other elements are electrically connected in series.
 11. An electroacoustic transducer comprising a rigid housing having a sound transparent window therein, a vibratile plate positioned adjacent said window, said plate being divided by deep grooves into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said vibratile plate being substantially circular, said grooves including two concentric circular grooves, the diameter of the smaller circular groove being approximately 50 percent to 60 percent of the diameter of the larger circular groove, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, each of said transducer elements being mounted to Drive an individually associated one of said islands, and means for electrically joining said transducer elements to drive said individual islands in a predetermined relationship with respect to each other.
 12. The invention of claim 11 and means for driving the piston elements lying within the area bounded by the smaller circular groove at an amplitude which is greater than the amplitude of elements lying in the region between said concentric grooves.
 13. An underwater electroacoustic transducer comprising a housing structure including a peripheral wall portion terminated at one side by a resilient sound transparent window exposed to surrounding water, a vibratile plate portion mounted to drive sound waves through said window and into said water, said vibratile plate having a linear transverse dimension which is greater than the wavelength of sound radiated into the water at the operating frequency of said transducer, grooves in one surface of said plate dividing said plate into a plurality of acoustically separated islands, each of said islands having individual transverse dimensions which are greater than the thickness of said islands, each of said islands forming a vibratile piston element movable as an integral unit, said islands being held together mechanically by the thin weblike structure remaining beneath the bottoms of said grooves, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, means for attaching said transducer elements in individually associated relationship to drive said separated vibratile piston elements, and means for electrically connecting said transducer elements to operate at a fixed relationship with respect to each other.
 14. The invention in claim 13 wherein said vibratile plate portion is sealed to said peripheral wall portion by said resilient material.
 15. The invention in claim 14 wherein said resilient material embraces said plate and completely covers the radiating surface of said vibratile plate.
 16. An underwater electroacoustic transducer comprising a housing structure including a peripheral wall portion terminated at one side by a resilient sound transparent window exposed to surrounding water, a vibratile plate portion mounted to drive sound waves through said window and into said water, said wall portion and said resilient window being a single unitary housing structure, said plate being suspended adjacent said resilient window, a coupling liquid completely filling said housing structure, said vibratile plate having a linear transverse dimension which is greater than the wavelength of sound radiated into the water at the operating frequency of said transducer, grooves in one surface of said plate dividing said plate into a plurality of acoustically separated islands, each of said islands forming a vibratile piston element, said islands being held together mechanically by the thin weblike structure remaining beneath the bottoms of said grooves, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations, means for attaching said transducer elements in individually associated relationship to drive said separated vibratile piston elements, and means for electrically connecting said transducer elements to operate at a fixed relationship with respect to each other.
 17. An electroacoustic transducer comprising a housing having a sound transparent resilient wall portion with at least one linear dimension which is larger than the wavelength of sound radiated by said transducer when being driven at its frequency of operation, a plurality of grooves recessed into said vibratile wall portion for dividing said vibratile wall portion into a plurality of pistonlike vibratile surfaces, a plurality of electroacoustic transducer element means, each of said transducer elements having a mass element held against and separated from said housing by a low acoustic impedance material, means for attaching said electroacoustic transdUcer element means in operable relationship to individually associated ones of said pistonlike vibratile surfaces, and electrical conductor means for interconnecting said electroacoustic transducer element means.
 18. In combination in an electroacoustic transducer, a housing structure having a peripheral portion and a resilient end portion sealed to said peripheral portion to form a generally cup-shaped unit, the external area of said end portion being characterized in this that it presents a smooth and uninterrupted surface to the medium into which the transducer is operating, a vibratile plate, the inner surface of said end portion being adjacent one side of said vibratile plate, the other side of said plate being provided with a matrix of deep grooves which divide said vibratile plate into a plurality of acoustically separated vibratile piston elements held together by the thin weblike grid structure remaining at the bottoms of said deep grooves, each of said piston elements being thinner than its narrowest transverse dimension, a plurality of transducer elements for converting electrical signals into corresponding mechanical vibrations attached in operable relationship to drive individual ones of said separated vibratile piston elements, and means for interconnecting said plurality of transducer elements to form electrical operating units.
 19. In combination in an electroacoustic transducer, a complex vibratile piston comprised of piston islands elements joined by an undercut web, each of said islands having a thickness which is less than the average transverse dimension of said islands, a plurality of electroacoustic transducer elements for converting electrical energy into mechanical energy, said elements being exposed to hydrostatic pressures surrounding said transducer, a plurality of inertial mass elements, means for attaching said mass and transducer elements in individually associated operable relationship, each of said transducer elements being mechanically bonded between individually associated piston islands elements and individually associated ones of said inertial mass elements so that axial vibrations of said elements may take place during vibration of said transducer elements, a plurality of stress means free of rigid attachments to a supporting structure for applying a predetermined compressive mechanical stress along a vibrational axis of individually associated ones of said transducer elements, each of said stress means including a rigid washerlike plate a pair of convex spring washers for neutralizing said hydrostatic pressures, and mechanical fastening means for applying a compressive force from said rigid washer through said spring washers, to the inertial mass element and along the vibrating axis of said transducer element.
 20. The invention in claim 19 characterized in that the transducer element is in the form of a hollow cylinder and further characterized in that said mechanical fastening means is a bolt which passes through the hollow cylinder. 